Microstructures and dislocations in the stressed AZ91D magnesium alloys

The microstructures and the dislocation arrangements in the die cast AZ91D magnesium alloy as well as the stressed alloys have been investigated using transmission electron microscopy, high-resolution transmission electron microscopy and energy dispersive X-ray (EDX) analysis. Besides the dominant α-Mg and β-Mg17Al12 phases, Al8Mn5 and Mg5Al phases have also been found and studied in the alloy. Dislocation pile-ups have been found in the stressed AZ91D alloys for the first time. They are confined in the slip planes and piled up against the grain boundaries. The dislocation pile-ups increase with deformation till about 2.1% and then remain almost identical even deformed to higher degree. The dislocations pile-ups in the AZ91D magnesium alloy are found to be beneficial to the resistance to stress corrosion cracking of the alloy, and thereby beneficial to the mechanical properties of the alloy. Dislocation networks are found to increase with deformation in all cases. The dislocation networks have also been found in the β-Mg17Al12 phase as well as in the matrix in the deformed AZ91D magnesium alloys.